Flow transducers are typically used to measure the mass flow rate of fluid through a conduit. Various mechanical mass flow transducers, which monitor fluid induced movement of mechanical components, are known in the art and provide rough measurements of the mass flow rate of a fluid. Electrical transducers are known to provide more accurate measurements of fluid flow. Electrical transducers of the thermal type typically rely on one or more temperature-sensitive, resistive elements disposed typically around the conduit. These latter types of transducers are based upon a well known relationship that the rate of heat transfer to a fluid in a laminar flow channel from the walls of the channel is a rather simple function of the temperature difference between the fluid and the walls of the channel, the specific heat of the fluid and the mass flow rate of the fluid within the channel. Since the specific heat of a gas does not vary greatly with pressure or temperature, a thermal mass transducer calibrated for a particular gas will give true mass flow readings over a wide range of operating conditions.
Thermal mass flow transducers therefore include one or more heating elements to transfer heat energy to a fluid stream flowing in a small laminar flow tube, sometimes known as a sensor tube. The heating elements are usually made of a metal alloy having a high resistance and high temperature coefficient of resistance. The sensor tube is usually a thin stainless steel tube, and the elements are wound tightly around the outside of the tube to provide effective heat transfer to the fluid without disturbing the fluid flow within the tube. The high temperature coefficient makes these heating elements also very good devices for sensing the temperature of the tube, and they are often employed in that double capacity. For clarity, such double duty heating/sensing elements will be referred to herein as "thermal elements". These thermal elements are well known and are described, for example, in U.S. Pat. Nos. 4,464,932, entitled THERMAL MASS FLOWMETERING and issued to Ewing; and 4,984,460, entitled MASS FLOWMETER and issued to Isoda.
While a one element fluid flow transducer has been described in U.S. Pat. No. 5,142,907 (Hinkle), thermal fluid flow transducers have tended to develop into two basic varieties, which may be designated the differential voltage variety and the absolute voltage variety. In the differential voltage variety of flow rate transducer, such as disclosed in U.S. Pat. Nos. 3,851,526 (Drexel) and 4,548,075 (Mariano), two identical thermal elements surround a laminar flow tube in a symmetrical tandem arrangement, one element being upstream from the other. The temperature differential between the elements is used as the measure of mass flow. In one traditional arrangement, shown in FIG. 1 and referred to as the two-element, constant current, differential voltage type, a constant current electrical source feeds both elements in a series circuit arrangement.
In FIG. 1 the prior art thermal fluid flow transducer 10 measures the mass flow of a gas flowing through a sensor tube 22 from, for example, a reservoir 24 to a process chamber 26. For small flow rates, the sensor tube 22 is of capillary dimensions and the transducer measures the flow rate directly through the conduit 22. For larger flow rates, both the sensor tube 22 and a by-pass tube 28 couple reservoir 24 to chamber 26, as shown in FIG. 1. Sensor tube 22, and by-pass tube 28 each draw a fixed percentage of the total gas flow. In such a system, the total gas flow between reservoir 24 and process chamber 26 is determined by multiplying the flow measured through sensor tube 22 by a scale factor. Different ranges of mass flows can be sensed by such a device by switching between different sized by-pass tubes 28.
Transducer 10 is shown as the two element, constant current differential voltage type. Specifically, transducer 10 includes a bridge of four resistors, 12, 14, 16 and 18. Resistors 12 and 14 are standard electrical resistors, such as ceramic resistors, and are chosen such that R.sub.12 (the resistance provided by resistor 12) equals R.sub.14 (the resistance provided by resistor 14). Resistors 16 and 18 are thermal elements in the form of coils that have an electrical resistance as a function of their temperature, preferably the resistance of each coil increasing as a function of temperature. Resistors 16 and 18 are chosen such that their temperature coefficients are equal, i.e., at any given temperature R.sub.16 (the resistance of coil 16) equals R.sub.18 (the resistance of coil 18). Further, the resistors R.sub.12 and R.sub.14 need to match the resistors R.sub.16 and R.sub.18, both in resistance (at zero flow) and in their temperature coefficients, in order to provide a reliable circuit. An example of this type of transducer circuit is shown and described in the Isoda patent.
The thermal elements, resistors 16 and 18, are typically wrapped around the sensor tube 22 and heated to the same initial temperature above the ambient temperature forcing the same current through each resistor. For this purpose a constant current source 20 provides current to the bridge, and specifically to the resistors 16 and 18. When gas from reservoir 24, which is usually at ambient temperature, flows through sensor tube 22 (as shown in FIG. 1), the flowing gas has a cooling effect on coils 16, 18 and lowers their temperature as a function of mass flow. The flowing gas cools coil 16 more than coil 18 because coil 16 is disposed upstream from coil 18. Transducer 10 measures the mass flow rate of gas flowing through tube 22 by measuring the difference in temperatures between coils 16 and 18, i.e., by measuring the difference in resistances between the two. Thus, when no gas is flowing through tube 22, coils 16 and 18 are at the same temperature and therefore, R.sub.16 equals R.sub.18. Since R.sub.12 equals R.sub.14, the voltage at node 32 equals the voltage at node 34. When gas flows through tube 22, R.sub.16 drops below R.sub.18 due to the differential cooling effect. Therefore, the voltage at node 32 drops below the voltage at node 34. Operational amplifier 36 generates a signal indicative of the difference between the voltages at nodes 32 and 34. This signal is fed to mass flow controller 30 which determines the mass flow rate through conduit 28 and compares it to a set point (the desired flow rate). Controller 30 in turn controls valve 32 to selectively adjust the gas flow rate if the rate sensed by the transducer 10 is not equal to the set point.
The transducer shown in FIG. 1 has several disadvantages. First, the difference between the voltages at nodes 32 and 34 is typically very small, even when gas is flowing at a maximum rate. Measuring this small voltage difference is difficult and the measurement is very susceptible to noise. Further, since the voltage difference is very small, the difference can not be measured remotely as is often desirable. Rather, the voltage difference must be measured by equipment that is in close proximity to the bridge. Secondly, the output of this device is non-linear. Typically, linearization circuitry is required to calibrate such a device.
Another type of differential (voltage) sensing variety of flow rate transducer is described in U.S. Pat. No. 4,624,138 (Ono, et al.), which can be referred to as the two-element, constant temperature, differential type. This transducer uses a heat producing resistor, which is heated to a constant temperature, and two thermal elements in the form of temperature-sensitive resistors. The heat producing resistor is disposed in a region of the conduit, and the two temperature sensitive resistors are disposed so that one is upstream and the other is downstream from the heat producing resistor. When gas flows through the conduit, the gas conducts heat from the heat producing resistor to the down stream temperature sensitive resistor. By measuring the differential voltages across the temperature-sensitive resistors, this device calculates the mass of gas flowing through the conduit.
This transducer has several disadvantages. Since this device relies on a constant temperature process (because the heat producing resistor is heated to a constant temperature) the device is only useful in a limited range of environmental temperatures. This device has further disadvantages when it is used in connection with a by-pass conduit such as shown at 28 in FIG. 1 because such by-pass instruments can produce undersirable effects when used with constant temperature sensors. When mass flow transducers are used in combination with a sensor tube and a by-pass conduit, it is generally assumed that the mass of gas flowing in the sensor tube is a fixed percentage of the total gas flow. This assumption is only correct if the temperature of the gas in the by-pass conduit is fixed relative to the temperature of gas in the sensor tube. This is true because the viscosity of a fluid depends upon its temperature. So if the temperature of the by-pass conduit varies with respect to the temperature of the sensor tube, the mass of gas flowing in the sensor tube will not be a fixed percentage of the total gas flow. Since the Ono et al. transducer heats the gas in the sensor tube to a constant temperature and the gas in the by-pass conduit can fluctuate with the ambient temperature, this transducer requires temperature compensation equipment if it is to be used in conjunction with a by-pass conduit.
A third type of differential (voltage) sensing variety of transducer can be described as the two-element, floating temperature, differential voltage transducer. Such a transducer is described in U.S. Pat. No. 4,984,460 (Isoda). This device requires four temperature-sensitive resistive elements. Two are disposed around the conduit and two are disposed in the ambient air. The device requires the temperature-sensitive resistors that are disposed in the air to have the same values of resistance and the same temperature characteristics (i.e., the same temperature coefficient of resistance) as the temperature-sensitive resistors that are disposed around the conduit. Requiring four temperature-sensitive resistors rather than two, and requiring that their resistances and temperature coefficients of resistances be matched, makes the implementation of the circuit much more difficult, adding significantly to the cost of the device.
U.S. Pat. No. 4,464,932, entitled THERMAL MASS FLOWMETERING, issued to Ewing et al. describes an example of the absolute voltage type of transducer, in which three thermal elements are used. This transducer can be described as the three element, constant temperature, absolute voltage transducer. It suffers from the same disadvantages as described in connection with the two element, constant temperature, differential voltage type transducer, and in addition the zero point is less stable since the measurement is absolute rather than differential.